1. Field of Invention
The present invention relates to a process and a device for removing combustion pollutants, such as nitrogen oxides, carbon monoxide, particulate matter like soot, volatile organic compounds, and other hazardous air pollutants. In particular the present invention is designed to operate under high oxygen conditions that often occur in combustion gases.
2. Background
Combustion products often contain many substances that require removal before release to the environment. Among these pollutants are nitrogen oxides, carbon monoxide, particulate matter like soot, volatile organic compounds, and other hazardous air pollutants. In addition most combustion processes operate with considerable excess oxygen, usually from air, so that the combustion gases still contain much residual oxygen. A combustion gas containing more than six percent oxygen by volume is deemed to produce a high oxygen condition for removal of combustion products.
In particular it is economically desirable to remove all combustion pollutants with one pass through an appropriate device and not have to employ successive apparatuses to fully accomplish the needed pollutant removal. The subject invention accomplishes this with a two stage process employing the selective use of appropriate catalysts along with microwaves created as a radiofrequency energy field.
A particular difficult combustion pollutant is nitrogen oxides present in various forms and usually identified as NO.sub.x to incorporate NO, NO.sub.2, etc. Microwave reduction of NO.sub.x proceeds well in the presence of pyrolytic carbon, such as char and soot, provided the oxygen content of the gas is small, less than 6 percent. As the oxygen content of the gas exceeds this 6 percent level, the removal of NO.sub.x becomes less and less efficient. Cha has shown this removal for the low oxygen situation in U.S. Pat. Nos. 5,246,554; 5,256,265; 5,269,892; and 5,362,451; and the specifications of these patents are hereby incorporated by reference. The subject invention covers the high oxygen case.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is conveniently divided into ultrasonic, microwave, and optical regions. The microwave region runs from 300 Mhz (megahertz) to 300 GHz (gigahertz) and encompasses frequencies used for much communication equipment. A treatise of such information is presented by Southworth, Principles and Applications of Wave guide Transmission, Nostrand, N.Y., 1950, which is herewith incorporated by reference.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies, such as 915 MHz to 5000 MHz, particularly with respect to the common frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of the atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions at a high temperature, often more than 3000.degree. F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither condition is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating by raising the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are affected to a much lesser extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
As used above microwaves are often referred to as a form of catalysis when applied to chemical reaction rates. For instance, see Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.
Related United States patents using microwaves include:
______________________________________ U.S. Pat. No. Inventor Year ______________________________________ 4,545,879 Wan et al. 1985 5,087,272 Nixdorf 1992 5,246,554 Cha 1993 5,256,265 Cha 1993 5,269,892 Cha 1993 5,362,451 Cha 1994 5,277,770 Murphy 1994 5,423,180 Nobue et al. 1995 5,536,477 Cha et al. 1996 ______________________________________
Referring to the above list, Nixdorf discloses using a filter containing silicon carbide whiskers to remove particulate matter from a gas stream and then clean said filter with microwave heating. The subject invention is not just a filter.
Cha ('554) discloses removing gas oxides by adsorption on a char bed and then reduction by microwaves as two distinct steps. The subject invention uses a related process for the removal of soot and some NO.sub.x in the presence of microwaves as one of several stages.
Cha ('265) discloses removing gas oxides in a homogeneous mixture with soot carried out in a waveguide reactor. In contrast the subject invention does not actively employ homogeneous reduction involving soot and has a non-microwave stage.
Cha ('892) discloses pyrolytic carbon bed for removal of gas oxides using microwave catalysis. The subject invention does not employ collecting a pyrolytic carbon bed since soot is burned as it is collected.
Cha ('451) discloses a waveguide reactor to efficiently perform radiofrequency catalysis. The subject invention does not employ a waveguide reactor.
Murphy discloses reactivating plasma initiators using microwaves in the presence of oxygen which is checked by a methane conversion reaction, where such plasma initiators are, or contain, metallic catalysts. The subject invention has no connection with the plasma regime of gases but does employ conventional metallic catalysts.
Nobue et al. disclose a filter regeneration system for an internal combustion engine using microwaves. The subject invention is not just a filter.
Cha et al. ('477) disclose a pollution arrestor using a soot filter followed by catalytic sections, using only reduction catalysts, to remove various gaseous pollutants with the total assembly within a microwave cavity. This pollution arrestor with only a reduction catalyst does not perform satisfactorily under high oxygen conditions. Conversely the subject invention performs well under high oxygen conditions.